The present invention relates to the field of air velocity or flow sensors. Air velocity sensors that are commercially available are commonly of the single hot wire or thermistor type and are typically mounted on the end of a long probe for insertion into the air stream. The temperature drop and the associated change in electrical resistance caused by the cooling effect of the air stream is a measure of the air flow velocity. In these devices the elements are fully exposed to the air stream and so are susceptible to breakage and contamination. Their temperature change with air flow is quite nonlinear and the resulting electrical signal must be carefully linearized by the circuit. Further, they are quite expensive and not suitable for large scale mass production.
The related commercially available mass flow sensor commonly consists of a metal tube through which the air or other gas is passed, a transformer which resistively heats a segment of the tube which is mounted by two massive heat sinks attached to the tube, and two thermocouples attached to the tube symmetrically, one on each side of the hot segment midway between the center and the heat sinks. Air flow through the tube cools the upstream thermocouple and heats the downstream thermocouple and the difference between thermocouple voltages at constant transformer power input is a measure of the mass flow. This is a massive instrument with substantial power requirements. It is not usable in a large duct or even open flow applications. It is expensive and not mass producible on a large scale.
A need exists for an air velocity or mass flow sensor and associated circuitry with these characteristics: long, maintenance-free life, small size, low power, easy adaption to a wide variety of air flow applications, large signal output, and linear or easily linearizable response over a broad velocity range. In addition, it must be mass producible on a large scale and be low in cost.
The literature contains several examples of attempts to improve the flow sensing art with respect to these needs. These attempts, which are further discussed below, generally utilize silicon and its semiconductor properties, or a pyroelectric material. The attempts improve the state of the art in some respects, but remain deficient with respect to many of the characteristics desired in a modern flow sensor. The subject invention advances the state of the art toward the satisfaction of these needs to a substantially greater extent than any of the prior art. The most relevant prior art known to the applicants will now be discussed.
Huijsing et al (J. H. Huijsing, J. P. Schuddemat, and W. Verhoef, "Monolithic Integrated Direction-Sensitive Flow Sensor", IEEE Transactions on Electron Devices, Vol. ED-29, No. 1, pp. 133-136, January, 1982) disclose a flow sensor comprising a silicon chip with an identical diffused transistor temperature sensing element embedded near each upstream and downstream edge of the chip and a centrally located diffused transistor heater element to raise the chip to as much as 45 degree Centigrade above the air stream temperature. The upstream sensing element is cooled slightly more than the downstream sensing element under air flow conditions, and the temperature difference between the two sensor element transistors results in an electrical current difference between them, which when converted to a voltage difference is a measure of the air flow. The sensor elements must be located on opposite sides of the chip to achieve an appreciable temperature difference between them, and even so, in the air flow range up to 1000 feet per minute, the temperature differences are small, ranging from 0 to under 0.2 degrees Centigrade.
Van Putten et al (A. F. P. Van Putten and S. Middlehoek, "Integrated Silicon Anomometer", Electronics Letters, Vol. 10, No. 21 [October, 1974], pp. 425-426) disclose a silicon chip with an identical diffused resistor element embedded on each of four opposite sides of a chip. All resistor elements are self heated, thus raising the chip and its support substantially above the ambient air stream temperature. The resistors are operated in an electrical double bridge circuit. Under zero air flow, all elements are at the same temperature, and the double bridge is electrically balanced. Under air flow conditions, the upstream and downstream elements which are normal to the flow are cooled more than the side elements which are parallel to the flow. This temperature difference unbalances the electrical double bridge to give a measure of the air flow velocity.
Malin et al (K. Malin and A. Schmitt, "Mass Flow Meter", IBM Technical Disclosure Bulletin, Vol. 21, No. 8, January, 1979) disclose a large silicon strip with a diffused central heater resistor element centrally located between upstream and downstream diffused sensor resistor elements. This prior art is an analog of the commercially available hot tube mass flow meter. The upstream sensor element is cooled by air stream transport while the downstream element is heated, and the resulting sensor resistor temperature difference results in a voltage difference across the sensor elements which is a measure of air mass flow.
Rahnamai et al (H. Rahnamai and J. N. Zemel, "Pyroelectric Anomometers", paper presented at the 1980 International Electron Devices Society of IEEE, Washington, D.C., Dec. 8-10, 1980, pp. 680-684) disclose a thin slab of single-crystal, polished, and crystallographically-oriented lithium tantalate, having a length of 8 mm, a width of 4 mm and a minimum thickness of 0.06 mm, a deposited thin film of metal covering the entire back side of the slab, a front side centrally-located deposited thin film heater resistor element, and an upstream and downstream deposited thin film electrode located equally some distance from the heater element and equally spaced from it. As described in the literature, this slab is supported at its edges on a large modified screw head to provide an essential air flow channel below the slab. The upstream and downstream electrodes act as separate identical capacitor plates, the capacitors having as their other plate the back side electrode and, together with the back and side electrode, function as identical upstream and downstream temperature sensing capacitor elements. In operation, an a.c. voltage of low frequency, for example: 2 to 10 Hz, is applied to the heater element causing a periodic heating of that element above the ambient air stream temperature. The sensor elements are correspondingly periodically heated by thermal conduction, to a large extent through the pyroelectric lithium tantalate slab, and under zero flow conditions, each sensor element develops an identical periodic pyroelectric voltage caused by the temperature-dependent electrical polarization of the pyroelectric slab. The voltage difference between sensor elements is then zero at zero air flow. Under flow conditions, the upstream sensing element cools more than the downstream sensing element as stated in the literature, and the resulting sensor element temperature difference causes a voltage difference that is a measure of the flow.
As previously indicated, these attempts improve the state of the art in some respects, but remain deficient with respect to many characteristics desired in a modern flow sensor. The subject invention advances the state of the art toward the satisfaction of these needs to a substantially greater extent.